Network Working Group S. Cheshire
Internet-Draft D. Schinazi
Updates: 7050 (if approved) Apple Inc.
Intended status: Standards Track May 22, 2017
Expires: November 23, 2017
Special Use Domain Name 'ipv4only.arpa'
draft-cheshire-sudn-ipv4only-dot-arpa-07
Abstract
The specification for how a client discovers its network's NAT64
prefix [RFC7050] defines the special name 'ipv4only.arpa' for this
purpose, but declares it to be a non-special name in that
specification's Domain Name Reservation Considerations section.
Consequently, despite the well articulated special purpose of the
name, (at the time of writing) 'ipv4only.arpa' still does not appear
as one of the names with special properties recorded in the Special-
Use Domain Names registry.
This document formally declares the actual special properties of the
name, and adds similar declarations for the corresponding reverse
mapping names.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 23, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
1. Introduction
The specification for how a client discovers its network's NAT64
prefix [RFC7050] defines the special name 'ipv4only.arpa' for this
purpose, but declares it to be a non-special name in that
specification's Domain Name Reservation Considerations section.
Consequently, despite the well articulated special purpose of the
name, (at the time of writing) 'ipv4only.arpa' still does not appear
as one of the names with special properties recorded in the Special-
Use Domain Names registry [SUDN].
This document formally declares the actual special properties of the
name. This document also adds similar declarations for the
corresponding reverse mapping names.
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2. Specialness of 'ipv4only.arpa'
The hostname 'ipv4only.arpa' is peculiar in that it was never
intended to be treated like a normal hostname.
A typical client never looks up the IPv4 address records for
'ipv4only.arpa', because it is already known, by specification
[RFC7050], to have exactly two IPv4 address records, 192.0.0.170 and
192.0.0.171. No client ever has to look the name in order to learn
those two addresses.
In contrast, clients often look up the IPv6 AAAA address records for
'ipv4only.arpa', which is contrary to general DNS expectations, given
that it is already known, by specification [RFC7050], that no such
IPv6 AAAA address records exist. And yet, clients expect to receive,
and do in fact receive, positive answers for these IPv6 AAAA address
records that are known to not exist.
This is clearly not a typical DNS name. In normal operation, clients
never query for the two records that do in fact exist; instead they
query for records that are known to not exist, and then get positive
answers to those abnormal queries. Clients are using DNS to perform
queries for this name, but they are certainly not using DNS to learn
legitimate answers from the name's legitimate authoritative server.
Instead, these clients have, in effect, co-opted the DNS protocol as
an impromptu client-to-middlebox communication protocol, to
communicate with the NAT64/DNS64 [RFC6146][RFC6147] gateway, if
present, and request that it disclose the prefix it is using for IPv6
address synthesis.
It is this use of specially-crafted DNS queries as an impromptu
client-to-middlebox communication protocol that makes the name
'ipv4only.arpa' most definitely a special name, and one that should
be listed in IANA's registry along with other DNS names that have
special uses [SUDN].
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3. Consequences of 'ipv4only.arpa' previously being declared unspecial
As a result of the original specification [RFC7050] not formally
declaring 'ipv4only.arpa' to have special properties, there was no
mandate for any DNS software to treat this name specially.
Consequently, queries for this name had to be handled normally,
resulting in unnecessary queries to the authoritative 'arpa' name
servers.
Having millions of devices around the world issue these queries
generated pointless additional load on the authoritative 'arpa' name
servers, which was completely unnecessary when the name
'ipv4only.arpa' is defined, by Internet Standard, to have exactly two
IPv4 address records, 192.0.0.170 and 192.0.0.171, and no other
records of any type.
Also, at times, for reasons that are as yet unclear, the
authoritative 'arpa' name servers have been observed to be slow or
unresponsive. The failures of these 'ipv4only.arpa' queries result
in unnecessary failures of software that depends on them for DNS64
[RFC6147] address synthesis.
Even when the authoritative 'arpa' name servers are operating
correctly, having to perform an unnecessary query to obtain an answer
that is already known in advance can add precious milliseconds of
delay for no reason.
A more serious problem occurs when a device is configured to use a
recursive/caching DNS server other than the one it learned from the
network. Typically a device joining a NAT64 network will learn the
recursive/caching DNS server recommended for that network either via
IPv6 Router Advertisement Options for DNS Configuration [RFC6106] or
via DNS Configuration options for DHCPv6 [RFC3646]. On a NAT64
network it is essential that the client use the recursive/caching
DNS64 server recommended for that network, since only that DNS64
server can be relied upon to know the appropriate prefix(es) to use
for synthesizing IPv6 addresses that will be acceptable to the NAT64
server.
However, it is not uncommon for users to manually override their
default DNS configuration because they wish to use some other public
recursive resolver on the Internet, perhaps because they perceive
their preferred recursive resolver to be faster, more reliable, or
more trustworthy.
Another common scenario is the use of corporate VPN client software,
which overrides the default configuration to divert DNS requests to
the company's own private internal recursive resolver, because the
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local network's recursive resolver will typically be unable to
provide answers for the company's private internal host names.
Similarly, the company's private internal recursive resolver may not
be able to synthesize IPv6 addresses correctly for use with the local
network's NAT64 server, because it is unlikely to be aware of the
NAT64 prefix in use on the local network. It is clear that a single
recursive resolver cannot meet both needs. The local network's
recursive resolver cannot give answers for some company's private
internal host names, and some company's private internal recursive
resolver cannot give correctly synthesized IPv6 addresses suitable
for the local network's NAT64 gateway.
The conflict here arises because DNS is being used for two unrelated
purposes. The first purpose is retrieving data from a (nominally)
global database -- generally retrieving the IP address(es) associated
with a hostname. The second purpose is using the DNS protocol as a
middlebox communication protocol, to interrogate the local network
infrastructure to discover the IPv6 prefix(es) in use by the local
NAT64 gateway for address synthesis.
(Possibly this problem could be solved if we could force all NAT64
gateways to use the same Well-Known Prefix for IPv6 address synthesis
[RFC6052], but that would alleviate the need for 'ipv4only.arpa'
altogether.)
This document leverages this operational experience to update the
Domain Name Reservation Considerations section [RFC6761] of the
earlier specification [RFC7050] with one that accurately lists the
actual special properties of the name 'ipv4only.arpa' so that
software can legitimately make appropriate performance and
reliability optimizations.
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4. Security Considerations
Hard-coding the known answers for 'ipv4only.arpa' queries in
recursive/caching DNS servers reduces the risk of malicious devices
intercepting those queries and returning incorrect answers,
particularly in the case of recursive/caching DNS servers that do not
perform DNSSEC validation.
One of the known concerns with DNS64 [RFC6147] is that it interferes
with DNSSEC. DNSSEC may cryptographically assert that a name has no
IPv6 AAAA records, while at the same time DNS64 address synthesis is
contradicting this and claiming that IPv6 AAAA records do exist.
Section 3 of the DNS64 specification [RFC6147] discusses this:
... DNS64 receives a query with the DO bit set and
the CD bit set. In this case, the DNS64 is supposed
to pass on all the data it gets to the query initiator.
This case will not work with DNS64, unless the
validating resolver is prepared to do DNS64 itself.
The NAT64 Prefix Discovery specification [RFC7050] provides the
mechanism for the query initiator to learn the NAT64 prefix so that
it can do its own validation and DNS64 synthesis as described above.
With this mechanism the client can (i) interrogate the local NAT64/
DNS64 gateway with an 'ipv4only.arpa' query to learn the IPv6 address
synthesis prefix, (ii) query for the (signed) IPv4 address records
itself, and then (iii) perform its own IPv6 address synthesis
locally, combining the IPv6 address synthesis prefix learned from the
local NAT64/DNS64 gateway with the secure DNSSEC-signed data learned
from the global Domain Name System.
It is conceivable that over time, if DNSSEC is successful, the
majority of clients could move to this validate-and-synthesize-
locally model, which reduces the DNS64 machinery to the vestigial
role of simply responding to the 'ipv4only.arpa' query to report the
local IPv6 address synthesis prefix. In no case does the client care
what answer(s) the authoritative 'arpa' name servers might give for
that query. The 'ipv4only.arpa' query is being used purely as a
local client-to-middlebox communication message.
This approach is even more attractive if it does not create an
additional dependency on the authoritative 'arpa' name servers to
answer a query that is unnecessary because the NAT64/DNS64 gateway
already knows the answer before it even issues the query. Avoiding
this unnecessary query improves performance and reliability for the
client, and reduces unnecessary load for the authoritative 'arpa'
name servers.
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5. IANA Considerations
[Once published, this should say]
IANA has recorded the following names in the
Special-Use Domain Names registry [SUDN]:
ipv4only.arpa.
170.0.0.192.in-addr.arpa.
171.0.0.192.in-addr.arpa.
IANA has recorded the following IPv4 addresses in the
IPv4 Special-Purpose Address Registry [SUv4]:
192.0.0.170
192.0.0.171
6. Domain Name Reservation Considerations
6.1. Conventions and Terminology Used in this Section
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this section are to be interpreted as described in "Key
words for use in RFCs to Indicate Requirement Levels" [RFC2119].
6.2. ipv4only.arpa
The name 'ipv4only.arpa' is defined, by Internet Standard, to have
two IPv4 address records with rdata 192.0.0.170 and 192.0.0.171.
When queried via a DNS64 [RFC6147] recursive/caching server, the name
'ipv4only.arpa' is also defined to have IPv6 AAAA records, with rdata
synthesized from a combination of the NAT64 IPv6 prefix(es), and the
IPv4 addresses 192.0.0.170 and 192.0.0.171. This can return more
than one pair of IPv6 addresses if there are multiple NAT64 prefixes.
The name 'ipv4only.arpa' has no other DNS records of any type.
There are no subdomains of ipv4only.arpa. All names falling below
'ipv4only.arpa' are defined to be nonexistent (NXDOMAIN).
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The name 'ipv4only.arpa' is special to
(a) client software wishing to perform DNS64 address synthesis,
(b) APIs responsible for retrieving the correct information, and
(c) the DNS64 recursive/caching server responding to such requests.
These three considerations are listed in items 2, 3 and 4 below:
1. Normal users should never have reason to encounter the
'ipv4only.arpa' domain name. If they do, they should expect
queries for 'ipv4only.arpa' to result in the answers required by
the specification [RFC7050]. Normal users have no need to know
that 'ipv4only.arpa' is special.
2. Application software may explicitly use the name 'ipv4only.arpa'
for NAT64/DNS64 address synthesis, and expect to get the answers
required by the specification [RFC7050]. If application software
encounters the name 'ipv4only.arpa' in the normal course of
handling user input, the application software should resolve that
name as usual and need not treat it in any special way.
3. Name resolution APIs and libraries MUST recognize 'ipv4only.arpa'
as special and MUST give it special treatment. Regardless of any
manual client DNS configuration, DNS overrides configured by VPN
client software, or any other mechanisms that influence the
choice of the client's recursive/caching DNS server address(es)
(including client devices that run their own local recursive
resolver and use the loopback address as their configured
recursive/caching DNS server address) all queries for
'ipv4only.arpa' and any subdomains of that name MUST be sent to
the recursive/caching DNS server learned from the network via
IPv6 Router Advertisement Options for DNS Configuration [RFC6106]
or via DNS Configuration options for DHCPv6 [RFC3646]. Because
DNS queries for 'ipv4only.arpa' are actually a special middlebox
communication protocol, it is essential that they go to the
middlebox in question, and failure to honor this requirement
would cause failure of the NAT64 Prefix Discovery mechanism
[RFC7050].
4. For the purposes of this section, recursive/caching DNS servers
fall into two categories. The first category is the traditional
recursive/caching DNS servers that are in widespread use today.
The second category is DNS64 servers, whose purpose is to
synthesize IPv6 address records.
Traditional recursive/caching DNS servers SHOULD NOT recognize
'ipv4only.arpa' as special or give that name, or subdomains of
that name, any special treatment. The rationale for this is that
a traditional recursive/caching DNS server, such as built in to a
home gateway, may itself be downstream of a DNS64 server.
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Passing though the 'ipv4only.arpa' queries to the upstream DNS64
server will allow the correct NAT64 prefix to be discovered.
All DNS64 servers MUST recognize 'ipv4only.arpa' as special and
MUST NOT attempt to look up NS records for it, or otherwise query
authoritative DNS servers in an attempt to resolve this name.
Instead, DNS64 servers MUST act as authoritative for this domain
and generate immediate responses for all such queries.
DNS64 servers MUST generate the 192.0.0.170 and 192.0.0.171
responses for IPv4 address queries (DNS qtype "A"), the
appropriate synthesized IPv6 address record responses for IPv6
address queries (DNS qtype "AAAA"), and a negative
("no error no answer") response for all other query types.
For all subdomains of 'ipv4only.arpa', DNS64 servers MUST
generate immediate NXDOMAIN responses. All names falling below
'ipv4only.arpa' are defined to be nonexistent.
An example configuration for BIND 9 showing how to achieve the
desired result is given in Appendix A.
5. Traditional authoritative DNS server software need not recognize
'ipv4only.arpa' as special or handle it in any special way.
Recursive/caching DNS servers SHOULD routinely act as
authoritative for this name and return the results described
above. Only the administrators of the 'arpa' namespace need to
explicitly configure their actual authoritative name servers to
be authoritative for this name and to generate the appropriate
answers; all other authoritative name servers will not be
configured to know anything about this name and will reject
queries for it, as they would reject queries for any other name
about which they have no information.
6. Generally speaking, operators of authoritative DNS servers need
not know anything about the name 'ipv4only.arpa', just as they do
not need to know anything about any other names they are not
responsible for. Operators of authoritative DNS servers who are
configuring their name servers to be authoritative for this name
MUST understand that 'ipv4only.arpa' is a special name, with
records rigidly specified by Internet Standard (generally this
applies only to the administrators of the 'arpa' namespace).
7. DNS Registries/Registrars need not know anything about the name
'ipv4only.arpa', just as they do not need to know anything about
any other name they are not responsible for. Only the
administrators of the 'arpa' namespace need to be aware of this
name's purpose and how it should be configured.
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6.3. 170.0.0.192.in-addr.arpa and 171.0.0.192.in-addr.arpa
Since the IPv4 addresses 192.0.0.170 and 192.0.0.171 are defined to
be special, and are listed in the IPv4 Special-Purpose Address
Registry [SUv4], the corresponding reverse mapping names in the
in-addr.arpa domain are similarly special.
The name '170.0.0.192.in-addr.arpa' is defined, by Internet Standard,
to have only a single DNS record, type PTR, with rdata
'ipv4only.arpa'.
The name '171.0.0.192.in-addr.arpa' is defined, by Internet Standard,
to have only a single DNS record, type PTR, with rdata
'ipv4only.arpa'.
There are no subdomains of '170.0.0.192.in-addr.arpa' or
'171.0.0.192.in-addr.arpa'. All names falling below these names are
defined to be nonexistent (NXDOMAIN).
Practically speaking these two names are rarely used, but to the
extent that they may be, they are special only to recursive/caching
DNS servers as described in item 4 below:
1. Normal users should never have reason to encounter these two
reverse mapping names. However, if they do, queries for these
reverse mapping names should return the expected answer
'ipv4only.arpa'. Normal users have no need to know that these
reverse mapping names are special.
2. Application software SHOULD NOT recognize these two reverse
mapping names as special, and SHOULD NOT treat them differently.
For example, if the user were to issue the Unix command
"host 192.0.0.170" then the "host" command should issue the query
as usual and display the result that is returned.
3. Name resolution APIs and libraries SHOULD recognize these two
reverse mapping names as special and generate the required
responses locally. For the names '170.0.0.192.in-addr.arpa' and
'171.0.0.192.in-addr.arpa' PTR queries yield the result
'ipv4only.arpa'; all other query types yield a negative
("no error no answer") response. For all subdomains of these two
reverse mapping domains, all queries yield an NXDOMAIN response.
All names falling below these two reverse mapping domains are
defined to be nonexistent.
This local self-contained generation of these responses is to
avoid placing unnecessary load on the authoritative
'in-addr.arpa' name servers.
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4. Recursive/caching DNS servers SHOULD NOT recognize these two
reverse mapping names as special and SHOULD NOT, by default, give
them any special treatment.
5. Traditional authoritative DNS server software need not recognize
these two reverse mapping names as special or handle them in any
special way.
As a practical matter, only the administrators of the
'192.in-addr.arpa' namespace will configure their name servers to
be authoritative for these names and to generate the appropriate
answers; all other authoritative name servers will not be
configured to know anything about these names and will reject
queries for them as they would reject queries for any other name
about which they have no information.
6. Generally speaking, operators of authoritative DNS servers need
not know anything about these two reverse mapping names, just as
they do not need to know anything about any other names they are
not responsible for. Operators of authoritative DNS servers who
are configuring their name servers to be authoritative for this
name MUST understand that these two reverse mapping names are
special, with answers specified by Internet Standard (generally
this applies only to the administrators of the '192.in-addr.arpa'
namespace).
7. DNS Registries/Registrars need not know anything about these two
reverse mapping names, just as they do not need to know anything
about any other name they are not responsible for. Only the
administrators of the '192.in-addr.arpa' namespace need to be
aware of the purpose of these two names.
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6.3.1. ip6.arpa Reverse Mapping PTR Records
For all IPv6 addresses synthesized by a DNS64 recursive resolver, the
DNS64 recursive resolver server is responsible for synthesizing the
appropriate 'ip6.arpa' reverse mapping PTR records too, if it chooses
to provide reverse mapping PTR records. The same applies to the
synthesized IPv6 addresses corresponding to the IPv4 addresses
192.0.0.170 and 192.0.0.171.
Generally a DNS64 recursive/caching server synthesizes appropriate
'ip6.arpa' reverse mapping PTR records by extracting the embedded
IPv4 address from the encoded IPv6 address, performing a reverse
mapping PTR query for that IPv4 address, and then synthesizing a
corresponding 'ip6.arpa' reverse mapping PTR record containing the
same rdata.
In the case of synthesized IPv6 addresses corresponding to the IPv4
addresses 192.0.0.170 and 192.0.0.171, the DNS64 recursive/caching
server does not issue reverse mapping queries for those IPv4
addresses, but instead, according to rule 3 above, immediately
returns the answer 'ipv4only.arpa'.
In the case of a client that uses the 'ipv4only.arpa' query to
discover the IPv6 prefixes in use by the local NAT64 gateway, and
then proceeds to perform its own address synthesis locally (which has
benefits such as allowing DNSSEC validation), that client MUST also
synthesize 'ip6.arpa' reverse mapping PTR records for those
discovered prefix(es), according to the rules above: When a client's
name resolution APIs and libraries receive a request to look up an
'ip6.arpa' reverse mapping PTR record for an address that falls
within one of the discovered NAT64 address synthesis prefixes, the
software extracts the embedded IPv4 address and then, for IPv4
addresses 192.0.0.170 and 192.0.0.171, returns the fixed answer
'ipv4only.arpa', and for all other IPv4 addresses performs a reverse
mapping PTR query for the IPv4 address, and then synthesizes a
corresponding 'ip6.arpa' reverse mapping PTR record containing the
same rdata.
7. Acknowledgements
Thanks to Jouni Korhonen, Teemu Savolainen, and Dan Wing, for
devising the NAT64 Prefix Discovery mechanism [RFC7050], and for
their feedback on this document. Thanks to Geoff Huston for his
feedback on the draft, and to Erik Kline for pointing out that the
in-addr.arpa names are special too. Thanks particularly to Lorenzo
Colitti for an especially spirited hallway discussion at IETF 96 in
Berlin, which lead directly to significant improvements in how this
document presents the issues.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
DOI 10.17487/RFC3646, December 2003,
.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
.
[RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 6106, DOI 10.17487/RFC6106, November 2010,
.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, .
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
.
[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
.
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8.2. Informative References
[SUDN] "Special-Use Domain Names Registry",
.
[SUv4] "IANA IPv4 Special-Purpose Address Registry",
.
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Appendix A. Example BIND 9 Configuration
A BIND 9 recursive/caching DNS server can be configured to act as
authoritative for the necessary DNS64 names as described below.
In /etc/named.conf the following line is added:
zone "ipv4only.arpa" { type master; file "ipv4only"; };
The file /var/named/ipv4only is created with the following content:
$TTL 86400 ; Default TTL 24 hours
@ IN SOA nameserver.example. admin.nameserver.example. (
2016052400 ; Serial
7200 ; Refresh ( 7200 = 2 hours)
3600 ; Retry ( 3600 = 1 hour)
15724800 ; Expire (15724800 = 6 months)
60 ; Minimum
)
@ IN NS nameserver.example.
@ IN A 192.0.0.170
@ IN A 192.0.0.171
@ IN AAAA 64:ff9b::192.0.0.170 ; If not using NAT64 Well-Known Prefix
@ IN AAAA 64:ff9b::192.0.0.171 ; Place actual prefix here
Authors' Addresses
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
David Schinazi
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
USA
Phone: +1 669 227 9921
Email: dschinazi@apple.com
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